Electronics are increasingly desired for testing, measurement, control, actuation, and communication in downhole (i.e., in a wellbore) applications, such as measurement while drilling/logging while drilling (MWD/LWD) and directional drilling (or geosteering), wireline logging, coil tubing, slickline services, and the like.
Downhole environments are harsh. Accordingly, electronics in a downhole environment may experience high pressures, conductive fluids, corrosive chemicals, severe vibrations, and mechanical shocks that are in excess of their designed specifications. As a result, protective packages typically house the electronics to insure reliable operation.
A BHA typically includes a plurality of these protective packages (i.e., segments) connected end-to-end at segment joints. Each segment encloses electronics to protect them from the harsh environments and operating conditions. Typically, the segments are constructed from high-strength metal or metal alloys and have a tubular form to allow the BHA to be easily moved through a borehole regardless of the trajectory of the borehole.
Adjacent segments can be electrically connected to form a network allowing the electronics in each segment to share data and electrical power. Accordingly, the electronics inside the segments may be networked to communicate with each other. Due to geometrical and structural constraints of the segment joints, however, the possible network topologies are limited.
Owing to its simplicity and reliability, one popular network topology used for downhole communication is a bus network. The bus network includes a common electrical path, or paths (i.e., a bus) passing through each segment, connecting the segments, and terminated at each end.
Tools are electronic devices that operate to perform a particular function (e.g., formation measurement and evaluation, drillstring monitoring and geosteering, etc.). The tools may tap into the bus (i.e., tool bus) as communicating nodes to exchange data/power. A group of tools exchanging power and data via a bus are referred to collectively as a tool bus network (i.e., tool string). The simplicity of the tool bus network unfortunately also makes it prone to failure.
The tool bus network described thus far can be disabled if any part of the bus fails (i.e., single point of failure). Troubleshooting the point (or points) of failure in a disabled tool bus network is difficult because communication over a failed tool bus network is impaired or disabled. As a result, in situ (i.e., while downhole) and/or on the fly (i.e., during a downhole process) troubleshooting methods may be unavailable, and instead, the BHA must typically be extracted from the well bore for troubleshooting. Adding to the problem, the electronics in the extracted segments are not easily tested (i.e., probed) because they are typically sealed within pressurized chambers. In these cases, the BHA must also be dismantled piece-by-piece to troubleshoot the failure. To make matters worse, the extraction of the BHA does not guarantee successful trouble shooting for a few reasons.
First, the act of extracting the BHA may change or obscure the point (or points) of failure. Second, it may be impossible to emulate downhole conditions at the surface. As result, it can be very difficult to replicate a failure caused by a downhole condition while the BHA is at the surface. Third, experienced trouble-shooters may not be available at a drill rig site. In these cases, a failed tool string must be shipped back to a nearby repair and maintenance (i.e., R&M) location or technology center for troubleshooting.
A failure (i.e., fault) in a tool bus network can be disruptive, costly, and can lead to unwelcomed outcomes. For example, the time spent troubleshooting is non-productive time (NPT) for the downhole application. Accordingly, a lengthy repair process and its excessive cost can lead to a negative customer response. In addition, the adoption and/or validation of new tools can be significantly hindered by the time/cost of such failures.
The tool bus network described thus far has additional limitations. The tool bus network is limited in its convenience and immunity to human error. For signal integrity, the tool bus network requires a proper termination at each endpoint of the bus to prevent reflections. Because each job for a drill rig may have different requirements, tools in a tool bus network are assembled into a particular configuration for each job (i.e., on an ad hoc basis). As a result, an added procedure of setting up terminations at the bus endpoints, according to a particular configuration, is required. In other words, permanently installing terminators in a tool bus network is not feasible.
For downhole measurements and test to be accurately analysed and interpreted, measurements of where each tool is located relative to the frontend drill bit are required. Typically, a tool string layout map is used for engineers or geophysicists to derive such information. In practice, a tool string layout map is generated manually by field personnel according to tool tracking records when the tool string is assembled. There is also additional work required to keep track of records integrity and updates so as to ensure a mistake-free process.
The tool bus network described thus far has a low channel efficiency. As multiple nodes share a common bus channel, multiplexing is imperative. Usually, time-domain multiplexing (TDM) is used in downhole design for its simplicity and resultant reliability. At one moment, TDM allows only one node to transmit signal to the bus. Signal propagates through the entire bus regardless how close the transmitting node is to the receiving nodes, which consequently lead to inefficient usage of the bus channel. Although frequency-domain multiplexing (FDM) and code division multiplexing (CDM) support multiple nodes transmitting simultaneously, the complexity of the schemes requires sophisticated hardware design which inevitably restrict their applications in downhole.
Efforts have been made to overcome some of the limitations of the bus network described above. For example, to address bus failures, U.S. Patent Publications 2017/0002640 and 2017/0059637 disclose sensing bus currents for an over current event. If an over current event is sensed, switches are used disable a portion of the tool string so that a different portion of the tool string can still operate. In both disclosures, the over current events result from short circuits. Short circuits, however, are only one mode of failure (i.e., fault) that the tool string may experience. The disclosures fail to address open-circuit failures and other faults, such as an intermittent bus connection, loss of a bus terminator, an erratic node response, and the like. In addition, neither disclosure teaches how to pinpoint the cause of the failure.